Low surface brightness -- how much does aperture help?

[akarsh]

Hi

Here is a somewhat newbie question.

What is the key requirement for observing objects with low surface brightness, such as dwarf galaxies and large, diffused nebulae? To me, it seems like dark, transparent skies and high contrast eyepieces and equipment are more important than aperture. What role does aperture play?

In my limited experience comparing a Takahashi 5" TOA apochromat with my 18" f/4.5 Obsession with a 31mm Nagler, the Takahashi's superior contrast made certain low surface brightness objects like Maffei-I obvious, whereas they were harder to observe in my 18". On the other hand, Stephan's quintet was easily observed in the 18", but hardly visible in the 5".

In a more recent comparison, I compared a 17.5" f/5 Discovery against a 12" Lightbridge and a 5" telescope. While there were hints of the dark lane of M 31 in the 5", the dark lane was much easier to detect in the 12" and was a bit more conspicuous in the 17.5" than in the 12". This being a matter of contrast too, it looks like contrast increases with aperture, but it seems like aperture is not the most important criterion.

Regards
Akarsh

[Ivan Maly]

Larger aperture allows me to apply (linearly) higher magnification without decreasing the surface brightness as viewed through the telescope, or to achieve (quadratically) higher surface brightness at the same magnification. The first effect or use of aperture seems the most significant, which could be explained by the fact that the quadratically higher optical surface brightness is perceived logarithmically by the our visual system. In practice (I observe mostly galaxies, which object class is comparatively uniform in its surface brightness distribution) I usually think what magnification I might need to detect a certain object or feature, and if my aperture will allow me to achieve this magnification without decreasing the surface brightness (again, as viewed through the telescope) too much. That said, expectations should be lower of larger-aperture amateur instruments that tend to be more poorly baffled.

[akarsh]

Thanks Ivan! That makes sense!

But what about contrast? If I define contrast naïvely as (Intensity of Object)/(Intensity of Background), then that never changes with aperture or magnification. So, from experience, this notion of contrast is not appropriate for our eyes. Maybe once the background is dark enough, the eye only cares how much the brightness of the object is...

Regards
Akarsh

[Ivan Maly]

It also cares about the apparent angular size. For detection (and enjoyment) a certain size is necessary, and bigger is better until framing becomes an issue.

In truth I almost doubt, in practical observing terms, that galaxies with a particularly low surface brightness exist. Galaxies are fairly uniform because of star formation and gravitational cohesion densities, and true low-luminosity galaxies with low star formation also tend to be dust-poor. The difficult galaxies that are commonly called low-surface-brightness are large on our sky, so magnification for their detection is not an issue with any instrument. This is why they respond differently, and don't respond as well, to increasing aperture - as far as their detection. Resolution of structure however becomes possible, and you can assemble the large galaxy visually from its parts. The difficult ones are those that lack the contrasty internal structure. Ample framing and widely averted vision may be necessary that may only be possible with a smaller aperture, but our visual system also "cares" about the overall brightness (with the same contrast).